Typical North American trucks have a full time 6×4 driveline. A 6×4 driveline typically means that there are 6 wheels on the truck and 4 of the wheels on two axles are driven. It is also typical that 6×4 drivelines have an interaxle differential and may or may not have optional wheel differential locks. The 6×4 system offers good traction in most conditions but suffers from spin losses and gear mesh inefficiencies.
6×2 systems (systems that have 6 wheels with 2 of them driving on a single axle) can improve spin losses and gear mesh inefficiencies but these systems are limited on tractive effort due to the single drive axle. The 6×2 system can utilize a wheel differential lock and an ability to shift weight to the drive axle under low traction conditions to improve tractive effort but it cannot equal the performance of the 6×4 system traction.
The 6×2 tandem is lighter than the 6×4 axle tandem but the suspension attachment for the non-drive tag axle is neither the same design nor the same roll stiffness as compared to the 6×4 tandem. What is needed is a tandem axle system that has the efficiency and lighter weight of the 6×2 system with the tractive effort and suspension interface of the 6×4 system.